Cordierite is a crystalline mineral that is desired for its physical properties, especially its relatively low rate of thermal expansion. Cordierite occurs naturally in certain rocks, but it is difficult to obtain substantially pure cordierite from this source and virtually impossible on a commercial basis.
Accordingly, the practice has been to synthesize cordierite. The crystalline structure of this mineral is generally taken theoretically to be a solid solution consisting essentially of the following oxides in the molar ratio: 2MgO.2Al.sub.2 O.sub.3.nSiO.sub.2, n being a whole number within the range of 5 to 8. As indicated, the cordierite lattice can accept 5 to 8 moles of SiO.sub.2 with respect to the ratio given to provide a thermal expansion only slightly higher than that of the molar ratio 2:2:5, respectively. Beyond the molar ratio of 2:2:8 crystal composition, no further silica is accepted and cristobalite inversion appears in the expansion curves; note "Thermal Expansion of Some Glasses in the System MgO--Al.sub.2 O.sub.3 --SiO.sub.2 " by Hummel and Reid, Journal of the American Ceramic Society, Volume 34, No. 10, October, 1951, page 319. Beyond the indicated saturation limit for silica, the presence of free silica is indicated as by changes in the thermal expansion curves.
Although, as indicated, cordierite is considered to have a basic chemical composition, 2MgO.2Al.sub.2 O.sub.3.nSiO.sub.2, there is evidence of limited solid solutions in cordierite toward the theoretical compound "Mg-beryl" (3MgO.Al.sub.2 O.sub.3.6SiO.sub.2), and also between cordierite and silica to the composition 2MgO.2Al.sub.2 O.sub.3.nSiO.sub.2, where n can be any whole number from 5 to 8. The complex nature of the proposed solid solutions and the long times that may be required to reach equilibrium in compositions of this type have resulted in conflicting opinions as to whether these solid solutions were stable or metastable at room temperature.
The crystal structure of cordierite is similar to the structure of the mineral beryl which consists of six-membered silicon-oxygen tetrahedron rings. Because of the differences in chemical composition between beryl and cordierite, the six-membered rings in the cordierite structure consist of one (AlO.sub.4).sup.-.sup.5 group and five (SiO.sub.4).sup.-.sup.4 groups. The differences between the two forms of cordierite, high or disordered cordierite and low or ordered cordierite, lie in the degree of order in the distribution of these aluminum-oxygen tetrahedra in the structure. High cordierite, with hexagonal symmetry, is found in nature as the mineral indialite and is most commonly associated with volcanic structures. Low cordierite has orthorhombic symmetry and is most commonly found in metamorphic rocks where the greatest degree of ordering in the cordierite crystal structure is achieved.
Modified cordierite structures also exist including a series of solid solutions between 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 and 2FeO.2Al.sub.2 O.sub.3.5SiO.sub.2, between 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2 and 2MnO.2Al.sub.2 O.sub.3.5SiO.sub.2, and ostensibly between 2FeO.2Al.sub.2 O.sub.3.5SiO.sub.2 and 2MnO.2Al.sub.2 O.sub.3.5SiO.sub.2. All these compositions maintain the cordierite crystal structure and also may display the same attractive physical and thermal properties as previously described for magnesian cordierite.
The term "cordierite-type" is used here and in the claims to include any and all of the cordierite crystalline structures previously described.
Cordierite may be synthesized either through a solid state reaction of metal oxides or by recrystallization from a glass. In either case, high cordierite always forms first. Some degrree of structural ordering of the alumina-oxygen tetrahedra can be achieved by prolonged heat treatment of high cordierite at elevated temperatures. In the formation of cordierite with the composition 2MgO.2Al.sub.2 O.sub.3.5SiO.sub.2, by recrystallization from a glass, there are three important reactions which occur. An endothermic reaction at about 800.degree.C indicates a point where relaxation in the glass structure occurs, thereby allowing ionic mobility and the formation of nuclei or sites for subsequent crystallization at higher temperatures. An exothermic reaction representing the formation of beta quartz solid solution crystal phase occurs at about 950.degree.C. It is believed that there is a replacement of part of the silicon atoms in the crystal lattice of beta quartz by aluminum atoms and the corresponding filling of the spiral-like vacancies with magnesium atoms. The formation of high cordierite at the expense of beta quartz solid solution and most of the remaining glass is detected as an exothermic reaction occurring at about 1020.degree.C. Heating to temperatures up to about 1425.degree.C, depending on chemical purity of the starting glass, ensures maximum cordierite formation for that particular glass composition.
In one process for synthesizing cordierite, the oxides, MgO, Al.sub.2 O.sub.3, and SiO.sub.2, or batch materials yielding these oxides upon firing, such as talc and clay or other aluminum silicates, are smelted and then quenched to a solid glass. A desired article may be shaped from the glass melt prior to a rapid cooling. The glass or preformed article is then reheated to a crystallizing temperature in which cordierite is formed in situ. Unfortunately, the entire mass of solid glass does not convert to cordierite, such that there is coexistence of crystalline cordierite and a glassy phase. The presence of the glassy phase militates against realization in the glass-ceramic the desirable properties of cordierite, the more glassy phase being present, the more serious becomes the problem. While the glassy phase when fluid tends to densify the cordierite, in the resulting glass-ceramic the crystalline cordierite and vitreous glassy phase compete to dominate its physical properties. Glass-ceramics with the lowest residual glass have also the lowest coefficients of thermal expansion.
Attempts have, therefore, been made to increase the amount of cordierite that crystallizes, and thereby reduce the amount of glassy phase formed, by adding nucleating agents to the glass-forming batch. Previously, the amounts added, however, have been relatively large, for example, in the case of titania of the order of at least 5 to as much as 10% by weight and in some instances as much as 20% based on the weight of the glass. While such additives may have increased the crystallization of cordierite, their presence in such relatively large quantities has acted as a diluent or poison to the cordierite insofar as realizing the desired physical properties is concerned. For instance, the rate of thermal expansion unduly suffers. Use of significant amounts of such agents may also promote formation of crystalline systems other than the desired cordierite-type glass-ceramic.
In this respect, prior nucleating agents have a built-in counter-effect which at least in part negates any improvement that may have been realized in increasing crystallization to cordierite.